This invention relates to a solid electrolytic capacitor using a flat plate-shaped valve action metal and, in particular, relates to a solid electrolytic capacitor with a large capacitance and a low ESL (Equivalent Series Inductance), using a conductive polymer as an electrolyte.
In recent years, reduction in size and increase in functionality of digital devices have been advanced. Accordingly, power supplies to be connected to the devices have also been required for adaptation to driving at high frequencies. Following it, noise countermeasures and smoothing of power supply voltages have become necessary. Thus, the roles of electrolytic capacitors in electronic circuits have been getting important. In these circumstances, there has been an increasing demand for solid electrolytic capacitors that are small in size, large in capacitance, and low in impedance regardless of the frequency range.
Generally, the impedance of a capacitor is determined by an ESR (Equivalent Series Resistance) in a frequency range lower than a self-resonant point, while, is largely affected by an ESL in a higher frequency range. In the case of a solid electrolytic capacitor, using as an anode a valve action metal such as aluminum, tantalum, niobium, titanium, or magnesium and using a solid conductive polymer as an electrolyte, it is possible to realize a low ESR equivalent to that of a multilayer ceramic capacitor. This is the characteristics obtained by the fact that the resistivity of the conductive polymer serving as the electrolyte is about 1/10 to 1/100 as compared with those of other kinds of solid electrolytes, such as manganese dioxide, and driving electrolyte solutions.
An aluminum solid electrolytic capacitor having a flat-plate element structure, which is one example of a solid electrolytic capacitor using a conductive polymer as an electrolyte, is formed, for example, in the following manner. At first, a dielectric film being an insulating film is formed by anodic oxidation at the surface of an aluminum base in the form of a flat plate-shaped porous aluminum metal serving as a valve action metal. Then, an insulator of an epoxy resin or the like is formed at a predetermined position on the dielectric film so as to divide the aluminum base into two regions and a solid electrolyte layer of a conductive polymer is formed in only one of the regions. Further, a graphite layer is formed on the solid electrolyte layer by screen printing or the like and a metal layer of a conductive paste is further formed on the graphite layer. In this manner, a cathode portion is formed. On the other hand, in the other region defined by the insulator, the aluminum base is exposed by stripping the dielectric film. In this manner, an anode portion is formed. The cathode portion and the anode portion are electrically connected to a cathode lead frame and an anode lead frame by bonding with a conductive adhesive and welding, respectively. In this manner, an external cathode terminal and an external anode terminal of a solid electrolytic capacitor are formed, respectively.
In the solid electrolytic capacitor using the conductive polymer as the electrolyte, since the ESR is low, the impedance in the low frequency range is sufficiently low. However, in order to apply this capacitor to a high-frequency driven circuit, it is necessary to simultaneously reduce the impedance also in the high frequency range. However, in the solid electrolytic capacitor, there has been a problem that the leading lengths of electrode wiring circuits inside the capacitor become long due to the internal structure of the element, which causes an increase in ESL so that the impedance in the high frequency range increases accordingly.
As a measure for solving this problem, there has been proposed a multi-terminal capacitor having a plurality of external electrodes. The multi-terminal capacitor is effective for reducing the ESL regardless of the type of capacitor. An electrode substrate is provided in which a plurality of external anode terminals and external cathode terminals are embedded as electrical contacts between anode and cathode portions of a capacitor and the exterior. Then, the anode and cathode portions of the capacitor are electrically connected to the external electrode terminals on the electrode substrate, thereby forming connection terminals to the exterior, respectively. With this structure, it is possible to provide an increased number of current loops formed between the external anode terminals and the external cathode terminals and thus to achieve a reduction in ESL in the high frequency range.
As a multi-terminal solid electrolytic capacitor using a conductive polymer as an electrolyte, there is an example disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2002-237431 (Patent Document 1). The structure of the capacitor disclosed in Patent Document 1 will be described with reference to FIGS. 1A and 1B. FIG. 1A is a perspective view of the solid electrolytic capacitor disclosed in Patent Document 1 and FIG. 1B is a longitudinal sectional view taken along line A-A in FIG. 1A. In FIGS. 1A and 1B, a solid electrolytic capacitor 21 is configured such that cathode portions are formed on both sides of an anode body 22 in the form of a flat plate-shaped base made of a valve action metal and a plurality of through holes are formed in the thickness direction to thereby lead anode terminals, connected to the anode body 22, to the surfaces of the cathode portions. On each side of the anode body 22, an anodized film 23, a solid electrolyte layer 25, and a cathode layer 26 are formed in this order and a capacitor structure is formed between the anode body 22 and the cathode layer 26. The through holes are formed in a lattice point pattern in the solid electrolytic capacitor 21 and regions of the anode terminals are provided in the lattice point pattern on the surface of each cathode layer 26 where a region serving as a cathode is spread over.
An anode lead portion 27 is formed in each through hole by press-fitting a valve action metal therein. Each anode lead portion 27 is in contact with the anodized film 23 being an insulator and is further in contact with the solid electrolyte layer 25 and the cathode layer 26 through an insulator layer 24. In the multi-terminal solid electrolytic capacitor disclosed in Patent Document 1, although there is no particular description about an electrode substrate, it is considered possible, when forming the capacitor into a product, to use an electrode substrate provided therein with a plurality of external anode terminals and external cathode terminals by disposing it on the upper or lower surface of the capacitor and connecting its external anode and cathode terminals to the anode lead portions 27 and the cathode layer 26, respectively, using a conductive adhesive or the like. Further, insulation by coating or casing is necessary on the side surfaces and the bottom surface of the capacitor and, further, it is also necessary to provide some structure for electrical connection between the cathode layers 26 on the upper and lower sides.
This solid electrolytic capacitor is fabricated in the following manner. At first, there is prepared a flat plate-shaped anode body 22 made of a porous valve action metal such as aluminum, tantalum, or niobium. Then, a plurality of through holes are formed in the anode body 22 in a direction perpendicular to the flat surfaces thereof by etching or machining. Then, an anode lead portion 27 made of a valve action metal of the same kind as that of the anode body 22 is embedded in each of the through holes by press-fitting. Then, the anode body 22 is anodized to form an anodized film 23 at each of the main surfaces thereof. Thereafter, on each anodized film 23, an insulator layer 24 is formed around each of the projecting anode lead portions 27 by the use of a photoresist or the like. Subsequently, a solid electrolyte layer 25 of a conductive polymer is formed on each anodized film 23 by chemical oxidation polymerization and then a cathode layer 26 composed of a graphite layer and a silver layer is formed thereon. Accordingly, the solid electrolytic capacitor 21 is configured such that the entire surface of the cathode layer 26 is exposed on each side of the capacitor 21 and the anode lead portions 27 each surrounded by the insulator layer 24 lie scattered side by side like islands in the surface of the cathode layer 26. According to this structure, by forming external electrode terminals into a multi-terminal structure, it is possible to obtain a solid electrolytic capacitor with shortened lengths of electrode wiring circuits and with a plurality of current loops. This makes it possible to realize a reduction in ESL and achieve an improvement in high frequency characteristics.
As an ESL reducing approach different from the multi-terminal capacitor with the plurality of external electrode terminals disclosed in Patent Document 1, a method has been examined that devises the shapes of an anode portion and a cathode portion. This method provides a structure in which, in leading paths of wiring circuits up to external electrode terminals in a capacitor, a region where an anode portion and a cathode portion face each other with an insulator interposed therebetween is provided as long as possible, thereby allowing current loops to be formed between the anode portion and the cathode portion facing each other. This corresponds to the case where a plurality of inductors are connected in parallel with each other between an anode and a cathode, thus leading to the effect of reducing the ESL of the capacitor on the whole. Since the external shape of a solid electrolytic capacitor is generally a flat-plate square shape, the shape of a cathode portion is advantageously rectangular for increasing the capacitance. In order to provide as long as possible a region where an anode portion and a cathode portion face each other with an insulator interposed therebetween when the shape of the cathode portion is rectangular, there has been proposed a double surrounding structure in which the insulator is formed so as to surround the peripheries of four sides of the cathode portion and further the anode portion is formed so as to surround the peripheries of four sides of the insulator.
As a solid electrolytic capacitor using a conductive polymer as an electrolyte and having the double surrounding structure as described above, there is an example disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2001-307956 (Patent Document 2). The structure of this capacitor will be described with reference to FIGS. 2A and 2B. FIG. 2A is a perspective view of the solid electrolytic capacitor disclosed in Patent Document 2 and FIG. 2B is a longitudinal sectional view taken along line A-A in FIG. 2A. In FIGS. 2A and 2B, a solid electrolytic capacitor 21 is configured such that a cathode portion is formed on the upper surface of an anode body 22 in the form of a rectangular, particularly square, flat plate-shaped base made of a valve action metal and a bottom anode lead portion 28 formed on the lower surface of the anode body 22 is drawn out from its side portions to the upper surface of the capacitor. On the upper surface of the anode body 22, an anodized film 23 is first formed and then a solid electrolyte layer 25 and a cathode layer 26 serving as the cathode portion are formed in this order. With this structure, a capacitor structure is formed between the anode body 22 and the cathode layer 26.
On the outer side of the solid electrolyte layer 25 and the cathode layer 26 serving as the cathode portion, an anode lead portion 27 is formed with an insulator layer 24 interposed therebetween. Since the bottom anode lead portion 28 is connected to the anode lead portion 27, the anode lead portion 27 is electrically connected to the anode body 22 through the bottom anode lead portion 28. The anode lead portion 27 is configured to surround the peripheries of four sides of the cathode portion through the insulator layer 24 so that there is formed a region where the anode portion and the cathode portion face each other with the insulator layer 24 interposed therebetween. Further, a protective layer 29 is formed on the side surfaces and the bottom surface of this capacitor element.
This solid electrolytic capacitor is fabricated in the following manner. At first, a sheet-like square anode body 22 made of a porous valve action metal such as aluminum or tantalum is subjected to anodic oxidation. In this manner, an anodized film 23 is formed at one surface (upper surface in the figure) thereof. Thereafter, a solid electrolyte layer 25 of a conductive polymer or manganese dioxide is formed and then a cathode layer 26 is formed on the surface thereof. Then, a bottom anode lead portion 28 greater in width than the anode body 22 is formed on the lower surface of the anode body 22 by a method such as metal deposition or sputtering or bonding of a metal layer. Then, an insulator layer 24 of an epoxy resin is formed so as to surround the side surfaces of the anode body 22, the anodized film 23, the solid electrolyte layer 25, and the cathode layer 26. Further, a metal layer of gold, copper, nickel, or the like is formed on the outer side of the insulator layer 24 by a method such as deposition or sputtering, thereby obtaining an anode lead portion 27. Finally, a protective layer 29 is formed on the side surfaces and the bottom surface of this capacitor element. The upper surfaces of the cathode layer 26, the insulator layer 24, the anode lead portion 27, and the protective layer 29 are configured to be flush with each other.
In this manner, as described above, in the solid electrolytic capacitor 21, the anode lead portion serving as the electrode drawn out from the inner anode body is formed so as to surround the peripheries of four sides of the square cathode portion with the insulator layer interposed therebetween. Therefore, it is possible to configure that the cathode portion and the anode lead portion face each other in this region. According to this structure, current loops can be generated between the anode portion (anode lead portion) and the cathode portion facing each other. Thus, it is possible to realize a reduction in ESL of the solid electrolytic capacitor.